In recent years, problems of exhaustion of energy resources and global environmental issues such as increasing CO2 in the atmosphere have driven demands for development of clean energy. In particular, utilization of solar batteries for photovoltaic power generation has been developed, been put into practical use, and been expanded as a new energy source.
An exemplary mainstream solar battery of such solar batteries is conventionally a dual-side electrode type solar battery. In the dual-side electrode type solar battery, a monocrystalline or polycrystalline silicon substrate has a light-receiving surface in which an impurity of conductive type opposite to that of the silicon substrate is diffused to form a pn junction. Electrodes are formed on the light-receiving surface and rear surface opposite thereto in the silicon substrate. In recent years, a so-called back-side electrode type solar battery is being developed in which both p type electrodes and n type electrodes are formed in the rear surface of a silicon substrate.
For cost reduction of raw materials, silicon substrates are getting thinner. However, as silicon substrates are getting thinner, solar cells are getting thinner, which disadvantageously results in cracks in cells caused upon operations for wiring the solar cells during fabrication of solar cell modules.
To solve such a problem, for example, Japanese Patent Laying-Open No. 2005-340362 (Patent document 1) proposes a method for wiring solar cells using a wiring substrate configured to include a base material, and wires formed on the base material.
As such, the utilization of a wiring substrate for connections of solar cells is proposed, but it has not been put into practical use yet due to the following problems.
First, in the case where electrodes of a solar cell and wires of a wiring substrate are connected to each other via a solder, when a general lead-free solder (Sn—Ag—Cu-based solder or the like) is used, the solder needs to be heated to around 250° C. Where the solder needs to be heated to such a high temperature to connect the electrodes of the solar cell and the wires of the wiring substrate to each other, stress is generated due to a difference in thermal expansion coefficient between the silicon substrate of the solar cell and the wire material of the wiring substrate during cooling after the high temperature heating, which results in a crack in the solar cell or decreased reliability in the connection therebetween such as removal of the solar cell from the wiring substrate.
Further, when connecting electrodes of a solar cell to wires of a wiring substrate using a solder, it is necessary to perform a step of aligning and positioning the solar cell on the wiring substrate and thereafter heating them using a reflow furnace. However, this step is complicated and a solar cell having a large surface area is likely to be displaced during the reflow, resulting in frequent occurrence of poor connection.
Furthermore, as indicated by Japanese Patent Laying-Open No. 2005-175436 (Patent document 2), it is considered to connect electrodes of a solar cell and wires of a wiring substrate using an ACF (Anisotropic Conductive Film) instead of a solder, but the ACF is expensive, which makes it difficult to use the ACF for a solar cell having a large surface area. In addition, the ACF have too large electric resistance for a large current to flow therein, resulting in decreased electric characteristics of the solar battery such as F.F (Fill Factor).                Patent document 1: Japanese Patent Laying-Open No. 2005-340362        Patent document 2: Japanese Patent Laying-Open No. 2005-175436        